[1] Characteristics of Power Distribution

PWR cores have the following features in power distribution;

• Enrichment zoning in fuel assembly is not necessary and enrichment is uniform, but there are some designs employing the enrichment zoning.

• The core radial power distribution can be flattened in the core design phase and no special adjustment is necessary during reactor operation.

• Only the core axial power distribution need to be adjusted during reactor operation.

These characteristics are attributed to the basic features of PWR core design which are no boiling in core, a canning-less fuel assembly (no channel box), cluster type control rods, and reactivity control by soluble boron concentration adjustment.

[2] Power Peaking Factor

The heat flux hot channel factor (Fq) and the nuclear enthalpy rise hot channel factor (FNh) are the basic design parameters concerned with the core power
distribution. Core radial and axial power distributions are flattened to meet the design limits of the parameters.

Nuclear Heat Flux Hot Channel Factor (Fq)

Nuclear hear flux hot channel factor is defined as the ratio of core maximum to average linear power density based on core design specifi­cations and given by the following factors

Fq =Ma. x{P(X, Y, Z)}xF$

P (X, Y,Z): the relative local power at position (X, Y,Z)

FU: the factor concerned with nuclear uncertainty (evaluated by a statisti­cal difference between calculations and measurements, usually about 1.05) The Engineering Heat Flux Hot Channel Factor (Fq)

The engineering heat flux hot channel factor is the factor to consider the effect of tolerances in fuel fabrication on the heat flux hot channel factor. Tolerances of pellet diameter, pellet density, enrichment, cladding thick­ness, etc are statistically combined to evaluate this factor. For example, it is 1.03 for uranium fuel and 1.04 for MOX fuel.

(iii) Heat Flux Hot Channel Factor (FQ)

The heat flux hot channel factor is defined as the ratio of the core maximum to average linear power density and given by Eq. (3.25).

TP — TpN v TpE

Fq — Fq X Fq

(iv) Heat Flux Hot Channel Factor (Fq(Z))at Core Height Z

The heat flux hot channel factor at core height Z is expressed as Eq. (3.26).

Fq (Z) = Max {P(X, Y, Z))xF"x

(v) Limits of Heat Flux Hot Channel Factor

Equation (3.27) is comprehensively used during normal operation and Eq. (3.28) is used for the limit of core detailed design and management.

Fqx P < 2.32

Fq (Z) xp< 2.32 XK(Z)

where P: The relative power

K(Z):the envelope curve function, for example, 2.32 x K(Z) in Fig. 3.54.

(vi) Definition and Limits of Nuclear Enthalpy Rise Hot Channel Factor The nuclear enthalpy rise hot channel factor is defined as the ratio of the maximum to average fuel rod power. It is limited in core design, for example, by relationships such as Eq. (3.29) which is used in evaluating DNBR.

Three Time-Burned Fuel

Twice-Burned Fuel

Once-Burned r uel

Fresh Fuel

Gd Gadolmia-Added Fuel

Fig. 3.47 Example of reload core fuel loading pattern with gadolinia-added fuel (Copyright Mitsubishi Heavy Industries, Ltd., 2014 all rights reserved)

F^h= 1.64{ 1 +0.3(1-P)} (3.29)